In unison with the recent start of experimental "hivision" (one form of HDTV developed by NHK) broadcasting in Japan, great efforts have been devoted for the development and improvement of associated equipment toward the future commercial broadcasting. Since the HDTV broadcasting entails a significantly large quantity of information which is more than five times the information quantity used in the current NTSC system of TV broadcasting, the signals involved therein have a so increased band width that it is difficult to transmit the signals with their band width unchanged using a single satellite broadcasting channel. Thus developed was the technique of subsampling the signals in accordance with a predetermined scheme for compression transmission, that is, MUSE system.
The receiver for receiving signals compressed by the MUSE system should have a device for restoring the input signals to the original form of signals, that is, MUSE decoder. The MUSE decoder is to complement subsampled-out data by interpolation between the available data and a change of the sampling frequency. Since signals in still picture regions and signals in motion picture regions, which both belong to video signals, are subject to different sampling modes at the encoder side, the MUSE decoder is designed to distribute the video signals between still and motion picture region signal processing paths where data interpolation and sampling frequency change are separately carried out. For the MUSE decoder, reference is made to "Nikkei Electronics", No. 433, Nov. 2, 1987, pages 189-212, "TV Technology", August 1989, pages 65-73, and Ninomiya, Yuichi, "MUSE - Hivision Transmission System", Dec. 1, 1990, published by Densi Joho Tusin Gakkai.
To illustrate the principle construction of the MUSE decoder, a video signal processing path is shown in FIG. 11. Although video signals include Y or luminance signals and C or color signals, the path for processing Y signals is generally illustrated herein and explanation is made to only Y signals in the following description unless otherwise stated.
In FIG. 11, an A/D converter 1 receives analog signals at the input, picks up data signals having a sampling frequency of 16.2 MHz as MUSE signals from the input signals, and deliver the MUSE signals to a MUSE decoder 2. The MUSE decoder 2 distributes the MUSE signals between a still picture region signal processing path 3 and a motion picture region signal processing path 4 at the same time. The still picture region signal processing path 3 includes an inter-frame interpolation means 5, a sampling frequency conversion means 6, and an inter-field interpolation means 7. The motion picture region signal processing path 4 includes an intra-field interpolation means 8 and a sampling frequency conversion means 9. In the still picture region signal processing path 3, the inter-frame interpolation means 5 serves to interpolate a signal between one field in a frame and one field in another frame, and the signal frequency is consequently changed from 16.2 MHz to twice the original, 32.4 MHz. Upon receipt of signals of 32.4 MHz, the sampling frequency conversion means 6 changes them to signals of 48.6 MHz, which are subject to inter-field interpolation in the inter-field interpolation means 7 for eventually delivering processed still picture signals to a still/motion picture signal mixer means 10. In the motion picture region signal processing path 4, on the other hand, the intra-field interpolation means 8 serves to create data intermediate data having a sampling frequency of 16.2 MHz within the same field through complement from the adjacent data, consequently forming signals of 32.4 MHz. Upon receipt of signals of 32.4 MHz, the sampling frequency conversion means 9 changes them to signals of 48.6 MHz for eventually delivering processed motion picture signals to the still/motion picture signal mixer means 10. The mixer means 10 is effective for mixing the processed still and motion picture signals under the control of a motion detection means (not shown), regenerating signals for forming an overall image.
The sampling frequency conversion means 6 and 9 of the still and motion picture region signal processing paths 3 and 4 may be constructed each as a digital filter comprising an FIR (finite-duration impulse-response) filter. This will be understood from the following description.
In general, the FIR filter, as seen from FIG. 12 showing its principle, includes delay elements D.sub.1, D.sub.2, . . . , D.sub.n typically in the form of shift registers, coefficient multiplier means M.sub.0, M.sub.1, . . . , M.sub.n, and an adder means A. Also available is the construction of FIG. 13 which is a modification or re-arrangement of the construction of FIG. 12. When the FIR filter shown in FIG. 12 or 13 is used as the sampling frequency conversion means 6 or 9 in the MUSE decoder, signals having a sampling frequency converted to 48.6 MHz can be derived as the addition output from the adder means A (or A.sub.1 through A.sub.n) by causing the FIR filter to operate at 97.2 MHz, the least common multiple between the input signal frequency 32.4 MHz and the output signal frequency 48.6 MHz, and determining proper coefficients to be multiplied in the coefficient multiplier means M.sub.0, M.sub.1, . . . , M.sub.n.
It is to be noted that since the frequency conversion means used in the aforementioned MUSE decoder have a specific integral ratio of 2:3 between its input frequency (32.4 MHz) and output frequency (48.6 MHz), the FIR filters used as the frequency conversion means can be reduced in cost by modifying the filters for parallel processing of signals for slowing down the operating speed of the filters. More particularly, since the actual input frequency to the filter operating at 97.2 MHz which is the least common multiple between the input and output signal frequencies is 32.4 MHz which is 1/3 of the filter operating frequency, among data of input signals, two of three data are zero, and since the output frequency is 48.6 MHz which is 1/2 of the filter operating frequency, no calculation is needed for one of two data. This suggests that an arrangement as generally shown in FIG. 14 can be used as the sampling frequency conversion means in the MUSE decoder.
The sampling frequency conversion means shown in FIG. 14 includes an FIR filter consisting of parallel arranged three gangs of filters F.sub.1, F.sub.2 and F.sub.3. A serial/parallel converter means 11 converts data signals of 32.4 MHz into parallel two successions of signals of 16.2 MHz which are delivered to the three gangs of filters F.sub.1, F.sub.2 and F.sub.3 operating at 16.2 MHz for parallel processing. Outputs of the three gangs of filters F.sub.1, F.sub.2 and F.sub.3 are sequentially selected by a switch means Sa in the form of a multiplexer for providing output signals of 48.6 MHz.
The principle sampling frequency conversion means for the MUSE decoder shown in FIG. 14 is embodied by a circuit arrangement as shown in FIG. 15. The serial/parallel converter means 11 is shown in FIG. 15 as including a shift register 11a operating at 32.4 MHz and shift registers 11b and 11c operating at 16.2 MHz. The three gangs of filters F.sub.1, F.sub.2 and F.sub.3 operating at 16.2 MHz have a common delay element section, but three separate sections of multiplier means and adder means. Signals across delay elements D.sub.1, D.sub.2, D.sub.3 and D.sub.4 are delivered at predetermined intervals to three groups of multiplier means M.sub.11, M.sub.12, M.sub.13, M.sub.14, M.sub.15 ; M.sub.21, M.sub.22, M.sub.23, M.sub.24 ; and M.sub.31, M.sub.32, M.sub.33, M.sub.34 where they are multiplied by respective coefficients. Outputs of multiplier means M.sub.11 -M.sub.15 are added by a first adder means A.sub.1, outputs of multiplier means M.sub.21 -M.sub.24 added by a second adder means A.sub.2, and outputs of multiplier means M.sub.31 -M.sub.34 added by a third adder means A.sub.3. Outputs of respective adder means A.sub.1, A.sub.2 and A.sub.3 are sequentially selected by a switch means Sa changing over at 48.6 MHz for providing output signals of 48.6 MHz.
As mentioned above, the MUSE decoder employs for both the still and motion picture region signal processing paths sampling frequency conversion means which can be embodied by digital filters. One most practical embodiment uses sampling frequency conversion means in the form of a digital filter as illustrated in FIG. 15 in each of the still and motion picture region signal processing paths. The sampling frequency conversion means in the form of a digital filter, however, requires many delay elements, many multiplier means and some adder means as understood from FIG. 15. Then the provision of two sampling frequency conversion means inevitably leads to an increased cost.